Another step towards supporting multiple readers.

There is just one thread.

The thread will always attempt to read more data since commit
2cf6b776, so we don't need to tell it
to continue.

The "seek_error" attribute will be eliminated soon.

More preparations to support multiple readers.

If a read error occurs, it is very unlikely that the InputStream will
ever recover.  Removing the code removes some code complexity which
just isn't worth it.  And it allows supporting multiple readers for
one buffer.

After seeking, a new read should be done.

Prepare to reuse it in another class.

Just like std::condition_variable, which however has no way to specify
the std::mutex directly.

Previously, a seek to the end of the file would cause an assertion
failure in SparseMap::Check() because the given offset was invalid.

Closes #453

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Instead of copying the offset from our `input`, copy the requested
offset to our `offset` attribute.

By the time Seek() finishes, our input's offset may have advanced
already, having read some more data, thus giving us a bogus offset.

This fixes spurious decoder failues (closes #320).